The tunnel-boring machine is howling and it's raining sludge when a project engineer informs us that our ride out of this damp, dark place has just broken down.

"Locomotive derailed," explains one of the engineers giving me a tour of the Deep Rock Tunnel Connector, a roughly 20-ft-diameter, 8-mile, combined-sewer-overflow tunnel under construction 250 feet below Indianapolis. 

We came down here during a shift change—they work 24 hours a day in this tunnel—and the little train that shuttles workers back and forth just slipped off its tracks.

"We can wait for it to get here, or we can start walking," the engineer adds. We look at each other briefly. I look at my watch—3:30 in the afternoon. We start walking. 

We had just spent about a half an hour on the TBM, which is louder than the front row of a rock concert. It sounds like about 12 rock concerts, I think. I was up for a little exercise that day—earlier, in the morning, I had swum a mile at the gym—but I was not mentally prepared for this trek.

"It's the one time in your life that you wish one leg were shorter than the other," says John Morgan, who manages the project for Citizens Water, the owner. I don't quite know what he means, but I soon find out.

The problem with walking a tunnel is that you can't always walk down the center of the tunnel—where it's flat. it's a dank, slippery environment. The walls of the tunnel are oozing with water, slime and petroleum. All these liquids swirl with sharp chunks of rock and sludge along the floor, where the partially submerged locomotive tracks lead into a foggy haze. At least the air circulating is fresh, I think.

Rather than risk getting our boots stuck in the goop, we walk down the sides of the tunnel—for more than two miles. I want to switch sides to relieve my ankles, but I also am trying to stay clear of the conveyor belt, so I stick to one side, mostly.

Anchored to one wall of the tunnel, to my left, the belt flies past my head, occasionally knocking debris my way. Every now and then, a metal splice in the belt slaps against the rollers, echoing down the tunnel in a bang…bang…bang that rings in my ears. I'm covered with sweat. "The people who work down here are tough," I say to myself.

We finally catch up with the locomotive. Workers have it jacked up and are using chain-falls to pry it back on the track. It isn't working. We keep walking.

After about 15 minutes, we hear a loud roar—a tow truck is coming. A small train with a crane mounted on it heads toward the stranded locomotive. But I don't have anywhere to go, and this train is coming right at me. The operator honks and slows down as I grab a piece of rebar sticking out of the wall—it is supporting utilities for the TBM. I lift my body to get out of the way. The train roars past.

After about an hour of carefully treading the wet rockface, dodging trains, doing my best to keep my camera dry and managing to save all my fingers from the rushing conveyor belt, we arrive at the main shaft. This may be the most fun I've had on a hardhat tour, and yet I've never been happier to step into a manbasket.

Read more about the Deep Rock Tunnel Connector project later this month at ENR.com.